Photosensing Characteristic of MOS Devices in Inversion Region and Enhancement of Sensitivity

In this thesis, the currents and illumination effect of MOS(p) and MOS(n) with ultrathin oxide, i.e. oxide thickness from 23 Å to 29 Å, along with MOS(p) with thin oxide, oxide thickness over 40 Å, are investigated. In chapter 2, by demonstrating the currents of MOS(p) and MOS(n) with and without illumination, the relationships between currents and oxide thickness are as followed: both the dark current and light currents of MOS(p) increases as the oxide thickness increases; on the other hand, the dark current of MOS(n) decreases as the oxide thickness increases and the light currents will have a minimum, which depends on the light intensity. From the previous works and the simulation of this work show that MOS(p) is influenced by Schottky barrier height modulation, while MOS(n) is controlled by the oxide electric field and may be deduced by Schottky Emission and Metal-Insulator-Semiconductor models. In chapter 3, by using the effect of lateral diffusion current, we discuss the characteristics of finger electrode. By having different gate electrode number under same length, it is shown that the dark current will increase as the electrode distance increases. It is speculated that the lateral electron concentration will decrease as the electrode distance decreases, which causes the current to decrease. On the contrary, under illumination, the lateral electron concentration is high. It will not affect the current when the electrode distances are ranging from 60μm to 10μm. The electrode number will dominate and cause the light current to increase. However, when the electrode distance is extremely small, i.e. 5μm, the lateral electron concentration will be shared and then decreases, which causes the light current to decrease. In chapter 4, MOS(p) with thin oxide thickness over 40 Å and having external voltage pulse to make the oxide breakdown is investigated. It is observed that the currents of MOS(p) with thin oxide will not saturate until it breakdown while MOS(p) with ultrathin oxide will get into deep depletion region and saturates. Thin oxide MOS(p) after breakdown will saturate in very small voltage, which is because the inversion layer at oxide breakdown point is deeper than origin. The electron movement is changed by this phenomenon and more electrons are induced into inversion layer, which causes the current increases. Under illumination, there is a hump of the current when the voltage is between accumulation and depletion region, and we use photovoltaic effect to explain the mechanism. Furthermore, this breakdown device has extremely high sensitivity.